Possibly the fact that the creator(s) of Android don't (directly) make any money out of the sales?
Every Apple phone sold = Money for Apple
Every Android phone != Money for Google
That's not to say that Apple shouldn't be worried, but unlike in the Mac vs. PC situation, their competition isn't a single company making profit off every unit sold. It's a mass of semi-co-operative, semi-competitive groups; many of which are helping themselves to the free Android software without giving anything back to the people who made it.
But every Android phone out there is another personal information hoover for Google that produces them money, so yes, they are profiting off of every unit sold that ships with the Google-branded applications provided the users use them and their locational services, which most users do because otherwise Android ships rather spartan to the point of urging you to use them.
Except that Google is slowly but surely having its wings clipped. So this idea that it will be able to leverage Android at will is misguided.
Combine all of that with the lack of online usage and engagement amongst Android users and everything isn't quite so rosy.
Shipping Google's applications (GMail, Maps, Google's store; maybe others) requires a license from Google. That probably is not free as in beer, either, but I find it hard to find definitive statements for that on the web. developer.android.com seems to only target software developers.
Also, some technologies used in Android are patented, and require payments to patent holders.
So, in practice, Android is not free for smartphone makers.
Full license, including the name, access to Play, all Google apps is between $10 and $20 per device. The license comes with a very good rate for every online search made from an Android phone or tablet.